chichiflyes:
Motivated by your findings, I pulled both my center (cap shroud) chainplates today. Same dimensions as you cited for your lower aft chainplates -- 37.0" overall and 35.5" on centers. I used stainless steel scouring pads, then "Spotless Stainless" to remove the minor surface discoloration/rust spots.
On inspection, I found a couple of areas of almost imperceptible surface pitting. Using a Dremel diamond fitting, didn't take much to reveal clean metal. Probably 1/64" was all I took off ... likely less. No sign of crevice corossion cracking was evident anywhere. So I have no concerns about continuing to use the center chainplates. Although when they go back in, I will reverse them. The bottom instead will be up. Let a new section reside in the deck core under the cover plate for the next few decades!
Interestingly, the pitting was further down, well inside the boat rather than at the deck core. And mostly was underneath what appeared to be very old remnants of clear packing tape. The kind with the fiber strands running through. I am thinking Hunter's fabricator used the tape to bundle several chainplates together for ease of shipping. And the installer didn't remove all the tape installing on the boat. I have read that SS crevice corrosion usual happens where salty water becomes trapped against the steel.
With the two chainplates out, I poked at the deck core with a small screwdriver. Was solid!
Only took a couple of hours to remove the two chainplates. After I re-install them, I will remove the others in sequence for a look-see. For peace of mind.